[Feature] Add ability to copy backups between backup buckets

[plkokanov]

Feature (What you would like to be added):
As part of the [Control Plane Migration] topic(https://github.com/gardener/gardener/blob/c00ec60c8423d5b95598e7a764d67f366f8c06b6/docs/proposals/07-shoot-control-plane-migration.md) (note that this is the revised GEP) we would like to add the functionality to copy shoot ETCD backups between seed backup buckets.

Motivation (Why is this needed?):
Currently, when a shoot cluster is migrated to a new seed, its backup is kept in the old seed's backup bucket. This means that seeds can have access to backup buckets which do not belong to them and also means that seeds can have BackupEntries which point to BackupBuckets of other seeds.

Approach/Hint to the implement solution (optional):
There is already a ready POC which was outlined in this issue: gardener/gardener#3875 and largely developed in this repo https://github.com/stoyanr/etcd-backup-restore/tree/cpm-poc/master

The plan is to first develop the backup copying as part of the Control Plane Migration "good case scenario" and only later on add functionality for "owner detection" which will be necessary for the "bad case scenario".

[plkokanov]

/assign @plkokanov